1. Field of the Invention
This invention relates to electrical power converters, and more particularly, to means for reducing electromagnetic interference or noise generated by power converters.
2. Background of the Invention
Electrical power converters are used to supply DC voltage to computer equipment. A typical power converter accepts line AC voltage and converts it into regulated DC voltage. Most computer systems require low voltage DC power, frequently at high current flows.
Modern power supplies employ switching-type regulators that are much smaller and less expensive to construct than earlier generation linear regulators. However, switching regulators generate electromagnetic interference (EMI) or noise because of the very fast rise and fall times of the current and voltage wave forms in the regulator.
The EMI noise generated by the switching regulator radiates from the device and is transmitted through the input and output conductors from the device. The present invention is concerned with noise conducted by the DC output circuit, e.g., the bus bars, that carry the current from the power supply to the operating computer voltage bus. EMI noise can lead to logic errors in the computer device and can cause interference with nearby electrical equipment. The Federal Communications Commission (FCC) and corresponding foreign agencies regulate levels of EMI radiated by an electrical device. It is, therefore, important to control the levels of EMI noise both for device reliability and to meet regulatory requirements.
The EMI transmitted through the bus bars can be classified into two types: differential noise between the positive and negative bus bars; and common noise between each bus bar and ground.
The prior art suggests several means of controlling EMI noise. Most common is the provision of power supply shielding to surround and contain EMI emissions from the power supply. Another frequent approach is to place noise filters on the AC input lines to the power regulator. Such an approach is described in High Frequency Switching Power Supplies: Theory and Design by George Chryssis, McGraw-Hill Book Company, 1984, pp. 199-200.
The use of shielding has the disadvantage of adding bulk to the power supply, thereby increasing its costs and assembly complexity. The use of AC line filters helps reduce EMI emissions in general, but is less effective in controlling noise transmitted to the computer device itself which can lead to logic errors.